1. Field of the Invention
This invention relates to an optical current transformer which is used to measure a relatively large current flowing through a high-voltage conductor such as a gas insulation opening and closing device by utilizing the Faraday effect when the polarization direction of light transmitted through glass under the influence of a magnetic field is rotated.
2. Description of the Related Art
An electric current can be measured by utilizing the rotational phenomenon manifested when polarized light is under the influence of a magnetic field, that is, the Faraday effect. The principle of the Faraday effect is that when polarized light is transmitted through certain types of glass, such as lead glass (hereinafter referred to as Faraday effect glass), which is under the influence of a current-induced magnetic field, the plane of polarization of the light is rotated by .theta.=V.multidot.H.multidot.L, where V is Verdet's constant, H is the magnetic field strength in the light travelling direction, and L is the glass length in the light travelling direction. The rotation .theta. is detected by a well-known method to measure the magnetic-field strength H, and the current flowing in the Faraday effect glass is calculated on the basis of the magnetic-field strength H.
In many cases, plural conductors are disposed at an ordinary current measuring location. Consequently, measurements are effected by magnetic fields caused by currents of conductors other than the conductor to be measured, which increases the measurement error. A known apparatus for solving the above problem by nullifying the effect of the currents of the conductors other than the target conductor is an optical current transformer in which an optical path surrounding a conductor is set up with Faraday effect glasses disposed about the optical path.
FIGS. 19A-19E are projectional views of a conventional optical current transformer, where FIG. 19A is a plan view, and FIGS. 19B, 19C, 19D and 19E are side views of FIG. 19A which are viewed from the right, upper, left and lower sides, respectively. In these figures, rod-shaped Faraday effect glasses 11, 12, 13 and 14 are arranged in a square form, and optical-path changing pieces 15, 16 and 17 are provided at three of the four corner positions of the square. These elements are fixed to a substrate 2 and to each other by an adhesive agent. A through hole 20 is provided at the center of the substrate 2, and a conductor (not shown) is inserted through the through hole. The two-dotted chain line with an arrow represents an optical path. Incident light beam a is transmitted through Faraday effect glass 11 to optical-path changing piece 15. The beam is totally reflected at a reflection plane at point b which intersects the optical path at a 45.degree. angle, and then totally reflected from another reflection plane at point c, so that the direction of the light beam is changed by 90.degree. upon exiting Faraday effect glass 12. Subsequently, in the same manner as described above, the light is transmitted along the optical path through optical-path changing piece 16, Faraday effect glass 13, optical-path changing piece 17, and Faraday effect glass 14, and is finally emitted from the right side of the Faraday effect glass 14 as emission beam h. Since the beam is transmitted while travelling around a current flowing through a conductor, the emitted beam h is polarized with respect to the incident beam a by a polarization angle which is proportional to the current. The reflection points of the optical-path changing pieces are represented by d, e f g, and symbols and Q representing a direction perpendicular to the plane of the drawing.
The manner for generating incident beam a leading to emitted beam h are not critical to the present invention, and thus a description and illustration thereof are omitted. In FIGS. 19A-19E, the optical path is illustrated as a square, and thus the Faraday effect glasses 11 to 14 have the same length. This is why the through hole 20 is formed in a circular shape. If the conductor is designed to be a straight-angle conductor and the through hole 20 is designed in a slit shape, the optical path is preferably designed in a non-square rectangular shape. Although the present description is directed to a square optical path, it may be generally applied to a rectangular shape.
In the optical current transformer described above, Faraday effect glasses 11, 12, 13 and 14 and optical-path changing pieces 15, 16 and 17 are fixed to substrate 2 with an adhesive agent. This is because any deviation in the relative positions of these elements will also cause a deviation in the optical path. When this deviation is beyond a permissible range, a light beam converted from incident beam a to emission beam h is not a normal emission beam and the optical current transformer cannot be normally operated. For example, with respect to the optical-path changing piece 15, if reflection points b or c are deviated from the reflection surface, the subsequent optical path is disturbed. Accordingly, the firm fixation of Faraday effect glasses 11, 12, 13, 14 and optical-path changing pieces 15, 16, 17 with an adhesive agent is required to prevent any positional deviation.
An epoxy resin or the like is generally used as the adhesive agent. Such an adhesive agent is an organic material having a coefficient of thermal expansion much different from that of the inorganic glass material used to form Faraday effect glasses 11, 12, 13, 14 and optical-path changing pieces 15, 16, 17. Accordingly, it has been determined that the following problems occur due to temperature variation:
1) Each of the optical-path changing pieces 15, 16, 17 is constructed by adhesively attaching two rectangular prisms to the transformer structure with an adhesive agent. Due to the different thermal expansion coefficients of the optical-path changing pieces and the adhesive agent, the two reflection surfaces cannot be kept perpendicular to each other when any positional deviation occurs between the two rectangular prisms due to temperature variation. Therefore, the state of the light beam, which is varied from a linearly polarized beam to an elliptically polarized beam through a first total reflection plane cannot be completely returned to the linearly polarized state through a second total reflection plane. Hence, the light is kept in the elliptical polarization state and thus an error occurs in the output signal.
2) The light beam is transmitted several times through the adhesive layers that are located between optical-path changing pieces 15, 16, 17 and Faraday effect glasses 11, 12, 13, 14 and between the two rectangular prisms which comprise the optical-path changing pieces 15, 16, 17. The adhesive layers will show anisotropy through repetitive thermal stress which occurs in the layers due to temperature variation, which, in turn, causes a phase shift in the two orthogonal components of the light beam, or light scattering, so that an error occurs in the output signal.
3) Due to the difference in thermal expansion coefficients between the adhesive agent and Faraday effect glasses 11, 12, 13, 14, the phase shift of the two orthogonal components of the light beam occurs in Faraday effect glasses 11, 12, 13, 14 due to so-called photo-elasticity when the temperature varies, so that an error occurs in the output signal.
As described above, these problems magnify the error of the optical current transformer, and thus heighten a possibility that the precision required for the optical current transformer cannot be kept.